The present invention relates to a direct-current motor for use in magnetic disk drives, video tape recorders, casstte tape recorders, and various other motor-operated devices for home, office and industrial use.
The recent trend of magnetic disk drives is toward a lower-profile configuration, and so is the trend of direct-current motors for driving such devices. However, direct-current motors are also required to have high-speed starting characteristics. Other devices such as information-related devices other than magnetic disk drives also have a tendency toward a lower profile, and direct-current motors used therein are required to be reduced in thickness and have high-speed starting characteristics.
FIGS. 1 and 2 of the accompanying drawings show a conventional direct-current motor for use in magnetic disk drives, the illustrated direct-current motor having a rotor assembly and a stator assembly which are disposed in radially confronting relation to each other. The rotor assembly, generally designated I, is composed of a rotatable shaft 1, a turntable 2 fixed to the rotatable shaft 1, an attachment boss 3 secured to the rotatable shaft 1 remotely from the turntable 2, a cup-shaped rotor yoke 4 attached to the attachment boss 3, and a drive magnet 5 fixed to an inner peripheral wall surface of the rotor yoke 4. The drive magnet 5 is magnetized radially perpendicularly to the rotatable shaft 1. The stator assembly, generally designated II, is disposed radially inwardly of the drive magnet 5 in spaced relation thereto. The stator assembly II is composed of a laminated stator core 6 having a plurality of circumferentially spaced teeth or pole pieces 6a and a plurality of radial yokes 6b interconnecting the pole pieces 6a, and stator windings 7 coiled respectively around the radial yokes 6b of the stator core 6. The stator assembly II is fixed to a housing assembly III composed of a pair of axially spaced bearings 8, 9 by which the rotatable shaft 1 is rotatably supported, and a housing 10 accommodating the bearings 8, 9 therein and supporting the stator assembly II. A printed-circuit board 11 is attached to the housing assembly III and serves to install a position detector and a speed detector thereon and connect the stator windings.
FIG. 3 of the accompanying drawings illustrates another prior direct-current motor in which a rotor assembly I and a stator assembly II are disposed in axially confronting relation to each other. The rotor assembly I comprises a rotatable shaft 1, a turntable 2 fixed to the rotatable shaft 1, an attachment boss 3 secured to the rotatable shaft 1 remotely from the turntable 2, a cupshaped rotor yoke 4 attached to the attachment boss 3, and a drive magnet 5 fixed to an inner peripheral wall surface of the rotor yoke 4. The drive magnet 5 is magnetized axially parallel to the rotatable shaft 1.
The stator assembly II is constructed of a stator yoke 6', a printed-circuit board 11 fixed to the stator yoke 6', and stator windings 7 fixed to the printed-circuit board 11. The stator assembly II is mounted on a housing assembly III composed of a pair of axially spaced bearings 8, 9 and a housing 10.
In the direct-current motors illustrated in FIGS. 1, 2 and 3, a signal indicative of the angular position of the rotor assembly I is detected by a Hall-effect device, for example, which controls a current flowing through the stator windings to rotate the rotor assembly I.
With the direct-current motor construction shown in FIG. 1, the air gap between the stator core 6 and the drive magnet 5 can be selected to be about 0.5 mm with high mechanical accuracy. Therefore, the operating point of the magnetic circuit is normally high at a permeance ranging from 5 to 10 G/Qe. Since ring-shaped anisotropic ferrite magnets are poorer in characteristics and much expensive than isotropic ferrite magnets, the drive magnet 5 normally comprises a ring-shaped isotropic ferrite magnet.
Where the motor of FIGS. 1 and 2 has a height greater than a certain level, the permeance is large and hence a large starting torque can be obtained, and the motor is manufactured inexpensively. However, where the motor is to be of a lower profile, any reduction in the motor height will result directly in a reduction in the thickness of the stator core 6 since coil end heights A, B of the stator windings 7 remain substantially unchanged. For reducing the thickness of a direct-current motor having a motor height of 10 mm and a stator core thickness of 3 mm so that the motor height will be reduced 10% to 9 mm, the thickness of the stator core 6 has to be reduced about 30% to 2 mm. This is disadvantageous in that motor regulation (ratio of change of rotation per unit torque, larger motors have smaller values of motor regulation), which is one of main characteristics representative of the volumetric efficiency of the motor, is doubled, and the volumetric efficiency of the motor is reduced to about half.
According to the motor arrangement of FIG. 3, the area of flux interlinkage on the stator windings 7 is much larger than that in the motor arrangement shown in FIGS. 1 and 2, the area of flux interlinkage remaining substantially unchanged even if the motor is reduced in thickness. As a consequence, unlike the motor shown in FIGS. 1 and 2, the motor of FIG. 3 is not subjected to a reduced volumetric efficiency due to a reduced motor thickness. However, since the air gap between the drive magnet 5 and the stator yoke 6' is large, the operating point of the magnet circuit is selected to be a small permeance ranging from 0.8 to 1.5 G/Qe, for drawing a maximum amount of energy from the magnet 5. The drive magnet 5 is normally composed of an anisotropic magnet, since the magnet 5 is axially magnetized, and a disk-shaped anisotropic ferrite magnet can be fabricated relatively inexpensively.
With the conventional motor constructions as illustrated in FIGS. 1, 2 and 3, where the drive magnet 5 comprises a rare-earth magnet having a large energy product, the flux linkage across the stator windings 7 is increased and the torque generated per unit current is increased, so that the starting characteristics and volumetric efficiency of the motor are improved. However, the cost of the motor is increased at a much higher rate.
In the motor shown in FIG. 1, the drive magnet 5 is secured to the inner wall surface of the rotor yoke 4 and may be small in volume, so that the rotor assembly I has a relativley small inertia. Where the motor of FIG. 1 is reduced in thickness, however, the starting characteristics become poorer or the time required to get the motor started is longer since the volumetric efficiency is reduced, that is, the produced torque is much smaller. This problem becomes quite serious in case the motor is incorporated in information-related devices. In the motor illustrated in FIG. 3, the produced torque is not so greatly reduced as by the motor of FIG. 1 when the motor is reduced in thickness. However, since the operating point of the magnetic circuit is essentially low, the torque generated per unit volume of the drive magnet 5 is small, and hence the volume of the drive magnet 5 has to be increased in order to produce a desired amount of torque. This results in a larger inertia of the rotor assembly I and poorer starting characteristics.
There is known anther direct-current motor, as disclosed by Japanese Patent Publications Nos. 48-6323 and 49-34082, which has an armature having radial winding slots defined in opposite surfaces of an annular main core and communiating with each other across an outer peripheral edge of the main core, and three sets of field magnets disposed annularly so that they confront the outer peripheral edge and opposite side surfaces of the armature core and have polarities arranged in the same manner in the radial direction. According to the disclosed direct-current motor, the hypothetical surface area of flux linkage on the stator core and the magnets are about twice that in the motor shown in FIG. 1. For achieving a maximum output with the same volume of the motor, the actual effective area of flux linkage on the pole pieces and the magnets ranges from about 1/2 to 1/3 of that in the motor of FIG. 1 with the space factor of the armature windings being taken into account. Although the actual effective area of flux linkage of the disclosed motor is better than that in printed motors, nevertheless the armature windings are difficult to form, and no substantial increase in the volumetric efficiency can be achieved by the disclosed motor arrangements. In order to accomplish the stator core of the disclosed shape, it has to be molded usually of a material such as soft ferrite, and therefore a high iron loss is caused, unlike the motor of FIG. 1 in which insulated silicon steel plates are employed. In use, the disclosed motor consumes a large amount of current, has a greatly reduced output efficiency, becomes heated, and thus is not suitable for use in portable devices powered by batteries. Another problem is that, since the magnets are required to be disposed in surrounding relation to the stator yoke in three directions with like polarities facing each other, it is difficult to assemble the magnets because of repulsive forces from like polarities. In the case of a rotatable magnet arrangement, it is difficult to hold the magnets, and no uniform air gap is ensured, and attractive forces between the upper and lower magnets and the stator yoke are subjected to variations. Where an axial thrust bearing is employed, the shaft tends to move upwardly or downwardly, and the motor suffers from vibrations during operation.
The prior direct-current motors have therefore failed to meet requirements as to volumetric efficiency, starting characteristics, input/output conversion efficiency, and other characteristics required by information-related devices.